For tires, for use in trucks and buses, to which heavy load is applied, attention is paid to stiffness in bead portions thereof. This is because deformation at the bead portions is increased under a load. Namely, the bead portions are deformed and deflected in the axially outward direction under a load. In the side portions of a tire, the greater the height from a bead base line is (near a portion having a maximal width), the greater the deformation is under a load. An end of a turned-up portion of a carcass ply is positioned near the mid-portion, in the radial direction, of a bead apex. The higher a position of the end of the turned-up portion is, the greater the movement of the end is. As a result, damage such as removal of the turned-up portion is likely to occur. Namely, durability of the bead portion is reduced. According to market research, much damage at side portions of tires occurs near the ends of the turned-up portions in general.
On the other hand, reduction in weight of tires for use in trucks and buses is highly required in the market. As a method for reducing a weight of a heavy duty pneumatic tire, reduction in thickness of the bead apexes may be selected. The reduction in thickness of the bead apexes causes reduction in stiffness of the bead portions. Deflection of the bead portion in a direction outward of the tire is increased under a load. Deformation of the bead portion is increased. As described above, the end of the turned-up portion of the carcass ply is positioned near the mid-portion, in the radial direction, of the bead apex. Accordingly, movement of the end of the turned-up portion is increased, whereby damage such as removal is more likely to occur. This is confirmed by a quantitative determination using a finite element method and a CT scan. A tire structure which does not reduce durability of bead portions even when the thickness of the bead apexes is reduced for, for example, reducing weight of tires, is strongly required.
A technique for solving the aforementioned problem has been suggested. The technique is associated with, for example, a radial tire for use in trucks and buses as disclosed in Japanese Patent No. 3643191. In this tire, side surface rubber portions of bead portions include curved concave portions with which curved convex portions at edges of flange portions (hereinafter, referred to as rim flange) of a rim are engageable. The curved convex portion of the rim flange engages with and fits into the curved concave portion of the bead portion when an internal pressure and load are applied to the tire. As a result, a contact pressure between the curved concave portion of the bead portion and the curved convex portion of the rim flange may become uniform. Further, a creep change amount in the side surface rubber portion may be reduced.
However, in the technique described above, a radius of curvature of the curved concave portion of the bead portion is set so as to have a value approximate to a value of a radius of curvature of the curved convex portion of the rim flange, such that the curved convex portion of the rim flange fits well into the curved concave portion of the bead portion. As a result, when the tire is under a load and the bead portion is deflected outward, distortion is likely to concentrate on a portion (near a position of the end of the turned-up portion of the carcass ply) of the bead above a position at which the convex portion of the rim flange fits into the bead. Therefore, movement of the end of the turned-up portion cannot be reduced.